Summary of Significant Accounting Policies (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Summary of Significant Accounting Policies
Cash and cash equivalents/money market funds	$ 97,608	$ 61,349
Cash and cash equivalents/money market funds, Fair Value	97,608	61,349	45,510	29,766
Cash and money market funds, Amortized Cost	97,608	60,369
Cash and money market funds, Fair Value	97,608	60,369
Cash, cash equivalents and marketable securities, Amortized Cost	198,702	191,545
Cash, cash equivalents and marketable securities, gross unrealized gains	13	18
Cash, cash equivalents and marketable securities, gross unrealized losses	(11)	(34)
Cash, cash equivalents and marketable securities, Fair Value	198,704	191,529
Cash, Cash Equivalents and Marketable Securities:
Marketable securities, Amortized Cost	101,094	130,196
Marketable securities, Gross Unrealized Gains	13	18
Marketable securities, Gross Unrealized Losses	(11)	(34)
Marketable securities, Fair Value	101,096	130,180
Corporate debt securities
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	46,893	71,669
Available for sale securities, Due in one year or less, Gross Unrealized Gains	4	3
Available for sale securities, Due in one year or less, Gross Unrealized Losses	(8)	(24)
Available for sale securities, Due in one year or less, Fair Value	46,889	71,648
Commercial paper obligations
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	6,641	15,237
Available for sale securities, Due in one year or less, Gross Unrealized Gains	8	13
Available for sale securities, Due in one year or less, Fair Value	6,649	15,250
Foreign government bonds
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	21,487	18,519
Available for sale securities, Due in one year or less, Gross Unrealized Gains	1	2
Available for sale securities, Due in one year or less, Gross Unrealized Losses	(1)	(1)
Available for sale securities, Due in one year or less, Fair Value	21,487	18,520
Asset-backed securities
Cash, Cash Equivalents and Marketable Securities:
Available for sale securities, Due in one year or less, Amortized Cost	26,073	25,751
Available for sale securities, Due in one year or less, Gross Unrealized Losses	(2)	(9)
Available for sale securities, Due in one year or less, Fair Value	$ 26,071	$ 25,742